Environmental Engineering Reference
In-Depth Information
Figure 5.6
Schematic illustrations of some of the main aspects of metal-oxygen reac-
tions [Ref. 4].
reaction will be governed by the rate of migration of either of the reactants or
both in a countercurrent direction through the film.
The solid state diffusional transport takes place by various processes such as
(1) diffusion through oxide lattice and (2) diffusion along oxide grain boundaries
and other easy diffusion paths, e.g., dislocation networks, microchannels, etc.
Furthermore, during scale growth, many additional phenomena and secondary
processes may occur. Depending on the growth mechanism, cavities and closed
pores may develop in the scale as well as in the metal. These features will, in
turn, affect transport processes through the scale.
It is also a common experience that large stresses may build up in the scale
and in the underlying metal. Such stresses may cause plastic deformation or
cracking of the scale. If cracking takes place in segments or in repeated manner,
